There are problems in the environment such as global warming due to the continuous use of fossil fuel. Moreover, the use of uranium causes problems such as radioactive contamination as well as the need for facilities for disposing radio active waste. Accordingly, there is a strong demand for alternative energy and various researches thereon have been conducted. One representative type of alternative energy is solar energy.
A solar cell device is a device, which directly produces electricity by using an optical absorbing material generating an electron and hole when a light is irradiated. In 1839, a French physicist by the name of Becquerel first discovered the photoelectron-motive force wherein a chemical reaction induced by light generates a current. A similar phenomenon was also discovered in the case of solids such as selenium. Thereafter, numerous researches on solar cell were continuously carried out in relation to inorganic silicone. This is because a silicone based solar cell with about 6% efficiency was developed for the first time in the Bell Research Center in 1954.
Such inorganic solar cell device consists of the p-n junction of inorganic semiconductor such as silicone. Silicone used for solar cells can be classified into crystalline silicone such as single crystalline or poly-crystalline silicone and amorphous silicone. The crystalline silicone has a better energy conversion efficiency compared to amorphous silicone when the solar energy is conversed into electrical energy. However, it has inferior productivity due to time and energy used for growing the crystalline. Amorphous silicone has a superior optical absorption, allows easy enlargement and has good productivity compared to crystalline silicone. But, it is inefficient in terms of facilities since, for example, vacuum processors are required, etc. Particularly, in case of the inorganic solar cell devices, there are problems since it is difficult to process and mold them. This is because the manufacturing cost is high and the device is manufactured in the vacuum condition.
Due to such problems, various researches on the solar cell device using the photovoltaic phenomenon of organic material (instead of silicone) have been attempted. The photovoltaic phenomenon of organic material refers to a phenomenon wherein when the light is irradiated on the organic material, the organic material absorbs photons to generate electron-hole pairs, said pairs being separated from each other and transferred to anode and cathode, respectively, in which the current is then generated by such flow of the electric charge. In other words, typically in organic solar cells, when the light is irradiated on the junction of the electron donor and electron acceptor material, the electron-hole pairs are formed in the electron donor and the electrons are transferred to the electron acceptor to produce separation of the electron-hole. Such a process is referred to as “excitation of the charge carrier by light” or “photoinduced charge transfer (PICT)” and carriers generated by light are separated into electron-hole and produce electrical power through the outer circuit.
When considering the fundamentals of physics, the output power, which is produced in all solar power generations including the solar cells, is regarded as a product resulting from the flow of photoinduced exiton generated by the light and driving force. In the solar cells, the flow is related to the current and the driving force is directly related to the voltage. Generally, the voltage of solar cells is determined by the used electrode material, the solar energy conversion efficiency is the value obtained by dividing the output voltage into the input solar energy, and the total output current is determined by the number of absorbed photons.
The organic solar cells that are prepared by using the optic pumping phenomenon of organic materials as described above, can be classified into the multi-layer solar cell device, which introduces the electron donor and electron acceptor layers between the transparent electrode and metal electrode, and the sing-layer solar cell introducing the blend of the electron donor and electron acceptor.
However, the solar cells using the typical organic material have problems in terms of energy conversion efficiency and durability. In this respect, the Gratzel (Gr) research team in Switzerland developed a dye-sensitized solar cell, which is a photoelectrochemical solar cell, by using dye as a photo sensitizer in 1991. The photoelectrochemical solar cell suggested by Gratzel, et al. uses an oxide semiconductor comprising the titanium dioxide of nano particles and photosensitive dye molecules. In other words, the dye-sensitized solar cell is a solar cell prepared by introducing electrolyte into the inorganic oxide layer such as titanium oxide wherein the dye is absorbed between the transparent electrode and metal electrode, and undergoing a photoelectrochemical reaction. Generally, the dye-sensitized solar cell includes two types of electrodes (photoelectrode and opposing electrode), inorganic oxide, dye and electrolyte. The dye-sensitized solar cell is environmentally friendly since it uses environmentally harmless material and has a high energy conversion efficiency of about 10%, which is second only to that of amorphous silicone solar cell of the existing inorganic solar cells. Further, its manufacturing cost is about 20% of that of the silicone solar cell. Thus, its high possibility for commercialization was reported.
The dye-sensitized solar cell, which is manufactured by using the photochemical reaction as described above, is a multi-layer cell device wherein the inorganic oxide layer in which dye absorbing the light are absorbed between the cathode and anode, and wherein the electrolyte layer that reduces electrons are introduced. The conventional dye sensitive solar cell device is briefly described below.
The dye-sensitized solar cell of the conventional multi-layered type can include, for example, the titanium oxide layer/electrolyte/electrode in which the substrate/electrode/dye is absorbed. More specifically, the lower substrate, anode, titanium oxide layer in which dye is absorbed, electrolyte layer, cathode and upper substrate are successively laminated from the lower layer. At this time, the upper and lower substrates are generally prepared with glass or plastic, the anode is coated with ITO (indium-tin oxide) or FTO (fluorine doped tin oxide), and the cathode is coated with platinum.
In view of the operating principles of the conventional dye sensitive solar cell device as constituted above, the dye absorbs the photons (electron-hole pairs) to form excitons. The excitons are transferred from the ground state to the excited state when the light is irradiated on the titanium oxide layer on which the dye is absorbed. As such, the election-hole pairs are separated from each other, the electrons are injected into the titanium oxide layer, and the holes are transferred to the electrolyte layer. If the external circuit is set up according to the above, then the electrons move from the anode to the cathode via the titanium oxide layer through the conducting wire to generate a current. The electrons in the cathode are reduced by the electrolyte and the excited electrons are continuously transferred to the generated current.
However, the general dye-sensitized solar cell devices have a high energy conversion efficiency, while suffering from safety problems such as the degradation of property due to the evaporation of solution, leakage of electrolytes, etc. Such problems constitute a great barrier of commercialization. Various researches have been carried out to prevent such leakage of electrolytes. Especially, the dye-sensitized solar cells using the semi-solid or solid electrolytes have been developed to enhance the stability and durability of the solar cells.
For example, Laid-Open Publication No. KR2003-65957 discloses a dye-sensitized solar cell including polyvinylidene fluoride dissolved in a solvent such as N-methyl-2-pyrrolidone or 3-methoxypropionitrile. The gel-type polymer electrolyte prepared by such method has a high ion conductance at room temperature, which is similar to the liquid electrolyte, but makes the preparation process of cells difficult since it has an inferior mechanical property and also has a lower liquid retaining property of polymer electrolyte.
Researches using electrospinning as a technique for preparing such polymer electrolyte membrane are vigorously carried out. Electrospinning was filed as a patent application by Formhals (an engineer in Germany) in 1934. The scientific basis for electrospinning was developed from the idea of Raleigh in 1882 that the electrostatic force can overcome the surface tension of liquid when the liquid falls down. The polymer fiber prepared by electrospinning is included in the ultra-fine nanotechnology and its worldwide market scale approaches about one trillion dollars in 2100. Generally, the nanofiber is defined as a fiber having a diameter of 20 nm to 1 μm and prepared by electro spinning, which spins the polymer in a low viscosity state as a fiber for a split second by the electrostatic force. Mass production is capable so as to have applications such as nano particles and filters, electrolytes for fuel cells, medical applications, etc. Its applications are expected to continuously grow.
The greatest advantage of nanofibers is that they have larger surface areas compared to conventional fibers. Such an advantage allows a nanofiber to be used as an effective material for a filter. Electrospun nanofiber can be also used for a protective clothing, an antibiotic wound dressing, drug delivery material, etc. However, such nanofiber can be prepared only by an electrospinning method and the fiber is formed irregularly. Thus, it is difficult to control the formation of the fiber. To overcome such problems, a method is considered, which gathers the electric field at one side by making the end of the rotating focusing plate to be sharp. Also, another method is considered that arranges the nanofiber in a direction during processes by using an interspacing focusing plate. The mutual interspacing is formed by an electric field near the focusing plate and electrostatic charge of the fiber. Thus, the produced fiber is prepared. Recently, nanofiber has been deemed as a biomedical implantable material since it has high porosity and large surface area. Accordingly, such characteristics can be helpful to adhesion, growth, proliferation of cells, etc. However, nanofiber fabricated by electrospinning has an inferior physical property since the improvement of strength can be barely obtained by molecular arrangement of the polymer itself. To overcome such problems, various process parameters are used in many researches directed to the preparation of nanofibers.
Reneker of the Akron University announced the nanofiber preparation of various polymer materials and the method of modulating influence factors by the electrospinning method. Further, Drexel University has prepared a nano composite material having improved mechanical properties by complementing carbon nanotube by the electrospinning method. Deitzel announced that as the concentration of polymers increases, the diameter of nanofiber and the diameter of fiber tend to increase by increasing the polymer concentration according to the power law relation. Doshi and Reneker announced that if the surface tension of polymer solution becomes smaller, then the bead in fiber can be reduced.
The researches on electrospinning process and the product development related to biological application and the polymer material, which is subjected to electrospinning, have been continuously carried out. The Commonwealth University brought success to the technology in which the nano-sized fine cellulose originally existing in the blood is fabricated to generate the flannel shape of bandage by using the electrospinning technology. Ethicon Inc. prepared a suture (PDS) in the form of mono-filament of PDO using p-dioxanon as a raw material. Woodward, et al. (1985) suggested that thermal treatment is required since the degree of crystallinity of the non-woven fabric prepared by electrospinning is remarkably inferior to that of the polymer before electrospinning Ignatious demonstrated that the medication can be instantaneously administered at any time by using the nanofiber, which was subjected to electrospinning MIT Material Processing Center performed a research on the scaffold for artificial organs and the University of Harvard conducted a research on the nanofiber using non-tissues. Rutledge at MIT ISN (Institute for Soldier Nanotechnologies) has manufactured the PCL scaffold by using the PCL nanofiber having a size of 0.5-10 μm and being subjected to electrospinning, and developed a nanofiber for treating damaged articular cartilages. Yarin (2004) of Israel suggested a new method wherein the polymer solution is put at a lower part and then subjected to spinning to the upper side by using the ferromagnetic suspension system instead of the conventional spinning method.
Researches directed to nanofiber manufacturing by using electrospinning were conducted by various national universities and research institutes. However, they are mostly dependedent on experiments, and the main topics of researches have been the characteristics and morphology of nanofibers as observed in the experiments when modifying the process parameters.
In case of the solar battery using solid electrolyte, the solvent is removed from the electrolyte solution to compensate for the reduced efficiency by the solvent. Then, the electrons, which are entered through the anode electrode, are easily reduced by using the hole conductor material in solid phase, wherein the dye is oxidized to flow the current.
A research relating to the solar cell using the solid polymer electrolyte without solvents was first attempted by the De Paoli group of Brazil in 2001. This group prepared a polymer electrolyte comprising poly(epichlorohydrin-co-ethylene oxide)/NaI/I2, and it is reported that it has about 1.6% of energy conversion efficiency at 100 mW/cm2. Thereafter, the Flaras group conducted a research in 2002 for improving the mobility of I−/I3− by adding titanium oxide nanoparticles to polyethylene oxide with high crystallinity to decrease the crystallinity of polymer. The Center for Facilitated Transport Membrane of KIST conducted a research for effectively applying the low molecular weight polyethyleneglycol (PEG) to the dye-sensitized solar cell using a hydrogen bond in 2004 and reported that the resulting energy conversion efficiency is about 3.5%.
Recently, the Flavia Nogueira group manufactured a solid dye-sensitized solar cell in the form of TiO2 nanotube by using poly(ethylene oxide-co-epichlorohydrin), which was synthesized with ethylene oxide and epichlorohydrin at the ratio of 84:16 as a polymer electrolyte, and reported the energy conversion efficiency of 4.03% in 2007.
There still exists a strong need in the art to develop solid dye-sensitized solar cell devices to overcome the problems described above without reducing the ion conductivity and damaging the solid form.